Power steering gear with oil filled capillary balancing slots

ABSTRACT

An integral power steering gear assembly includes a piston and cylinder combination having greater output efficiency through improved fluid balancing and cushioning of the piston at its ends. The improved cushioning utilizes oil capillary slots in lieu of previously proposed epoxy resin strips. The slots may be fed with high pressure fluid filled by feed grooves along the sides of the piston adjacent the output rack. The slots are designed to have a shallow depth, preferably 0.001 inch-0.0025 inch, which is sufficient to provide for high capillary attraction of the oil molecules to the slot. The trapped oil molecules prevent slip stick action, that is molecular interaction between the piston and cylinder wall, but at the same time, the oil molecules remain trapped or isolated within the walls of the slot thereby minimizing leakage from the slot. The operating oil is delivered with higher efficiency in the power steering unit by a bifurcated oil delivery passage communicating with the inlet slot of the piston. This provides fluidized balance and cushioning of the valve within the piston, and also prevents starvation of the high pressure feed grooves and capillary slots for balancing and cushioning the piston within the cylinder. Accordingly, improved reversibility of both the piston and the valve are obtained.

TECHNICAL FIELD

The present invention relates to integral hydraulic power steeringgears, and more particularly, to a power steering gear having oildistribution and cushion features for improvement of the efficiency ofoperation of the piston within the cylinder and the distributing valvewithin the piston.

BACKGROUND ART

The most efficient power steering gear assembly in production today isthe gear having a spool-type distributing valve within the piston. TheSheppard Power Steering System providing this basic concept is firstdisclosed and claimed in previous U.S. Pat. No. 3,092,083, Sheppard,which patent is exclusively licensed to the same licensee as theinvention covered by the present application. This basic designintroduced in the early 1960's has been improved upon several times. Oneof the main areas of recognized improvement is in the area of reducingthe frictional forces between the piston and the cylinder and/or thevalve and the valve bore within the piston and thereby improving thereversibility of the gear.

A major breakthrough improvement in this area is represented by theSheppard U.S. Pat. No. 4,088,063, issued May 9, 1978 and alsoexclusively licensed to the same licensee, R. H. Sheppard Co., Inc.,Hanover, Pennsylvania. This generation of gears was first introduced inthe middle 1970's and has proved to be highly efficient and effective inproviding greater steering efficiency. Truck manufacturers using thisgeneration of gears have been able to appreciably reduce the size andweight of the power steering unit and at the same time gain improvedefficiency to handle the larger and larger vehicles carrying the greaterand greater payloads of today.

In the gear built under the Sheppard '063 patent, the superiorreversibility (change or beginning of movement) of the valve within thepiston and the piston within the cylinder results from several uniqueconcepts. First, longitudinal fluid bearing grooves are provided alongthe side of the piston opposite the pressurized fluid input groove tocushion the piston and balance the opposing forces of the pressurizedoil delivered to the power steering gear. The oil between the piston andcylinder provides a separating film between the adjacent like metalsurfaces, thus preventing sticking action between the parts andminimizing wear. Additional lubricating ports (advantageously positionedto eliminate unwanted bleeding between the high pressure and the lowpressure hydraulic fluid) further reduce friction and generally enhancethe reversibility or break loose characteristic of the piston within thecylinder of the gear. Thirdly, repositioning of the guide pin for thevalve adjacent the input shaft end of the piston provides improvedreversibility of the valve within the piston by obviating the effect ofslight deflection of the parts during periods of high dynamic torquingof the steering gear. All of these features, and other features, arefully described and claimed separately, and in combination, in the '063Sheppard patent.

The desire to further improve the reversibility within the powersteering gear thereby bringing about even greater operatingefficiencies, lead me to invent the concept covered in the patentapplication, Power Steering Piston and Cylinder Combination with PlasticWear Surfaces, Ser. No. 843,976, filed Oct. 20, 1977 now abandoned. Thisprior invention disclosed and claimed the concept of providing plasticwear surfaces adjacent the ends of the piston to provide an alternativeto the fluidized balancing bearings of the '063 patent. The plasticbearing strips are formed of epoxy substantially spanning the distancebetween the ends of the output rack and the ends of the piston. Theconcept, which has worked well in use, is a viable alternative to solvethe friction problem and to prevent the related slip stick phenomenafrom arising. The concept relies on the differential molecular structurebetween the plastic and the metal cylinder wall to prevent slip stickeffect; i.e., the different materials prevent tearing and wearing of theparts in this area of highly concentrated frictional forces. In otherwords, the plastic serves to interrupt the surface of the piston so thatthe frictionally engaged surfaces at the cylindrical interface do nothave matching molecular configurations, thereby preventing the tendencyof the surfaces to match and interlock with the deleterious tendency toliterally tear away molecules of metal from one part to the other.

During the advanced test program covering the power steering gear withthe improvements of the '976 application, as an alternative to theimprovements of the '063 Sheppard patent, it was discovered that inaddition to the expected increase in efficiency over the originalSheppard '083 design, there was a discovery of an unexpected, evengreater efficiency; in the nature of 5-6% greater performanceefficiency. Upon close inspection, this was found to occur when thelevel of the plastics strips was set below the cylindrical surface ofthe piston. A further careful analysis of the parameters of theinvention, that was first brought about by this investigation, thus ledme to the present invention. Basically, I have discovered a way to takethe best features from the improved piston of the '063 patent and thepiston of my improvement patent application '976, and change the oildistribution on and around the piston and/or the valve, to come up witha power steering gear with even greater efficiency than originallyanticipated.

OBJECTIVES OF THE INVENTION

Thus, it is the first and primary objective of the present invention toprovide a power steering gear utilizing improved fluid distributionmeans for minimizing friction, cushioning the parts, improving thereversibility and hydraulically preventing slip stick effect withrespect to the piston and/or the valve.

It is another and specific object of the present invention to provide apower steering gear wherein plastic inserts at the ends of the pistonare not required.

It is another specific object of the invention to produce a powersteering gear having superior hydraulic valving and cushion meansreducing the friction and counterbalancing the force of the incoming oilof the inlet slot to the piston.

It is another and specific object regarding the valve of the steeringgear to provide delivery means feeding from the inlet slot that providesa balancing effect on the valve and insures feeding of oil to thebearing and cushioning means in a rapid and efficient manner so as toprevent starvation in this critical area.

It is still another specific object to the present invention to profideoil delivery means such that the travel time and the loss of pressureenroute to the slots is minimized, thereby assuring the full effect ofthe valving and cushioning means with improved reversibility at eitherthe start of steering in a static mode, or for the simple change ofdirection during movement of the vehicle.

It is another object of the present invention to provide the improvementin reversibility wherein the balancing of the spool valve in the centerof the piston is complimentary to the balancing of the piston within thecylinder housing of the gear.

It is another object of the present invention to provide oil filledbalancing and cushioning means in which the high pressure oil has highcapillary attraction within the slot whereby escape of the oil moleculesfrom the slot is prevented, thereby minimizing leakage in the gear.

DISCLOSURE OF THE INVENTION

The present invention is characterized by the provision of shallow oilcapillary slots at the ends of a piston of a power steering gear. Theseslots serve to balance the piston against the opposite hydraulic forceprovided at the inlet slot of the gear. The trapped oil molecules areretained by capillary attraction within the slots to minimize leakage.This synergistic result is due to the very shallow depth of the slots,preferably 0.001 inch to 0.0025 inch. In other words, the slots arepreferably sufficiently shallow to set up a high capillary attractionwithin the slots thereby providing trapping and blocking of the oilwithin the slots and preventing deleterious leaking. Slip stick actionand frictional wear are effectively prevented and efficiency of the gearis enhanced approximately 5-6% over previous designs. The slots areefficiently fed with high pressure oil from the pump in an improvedmanner. The feeding of the oil is through a bifurcated delivery passageat the valve within the piston. The shortest possible path to the slotsis thus followed for assuring that the slots are filled with oil at alltimes, especially at the instant when steering commences. At the sametime this arrangement provides for balancing of the valve within thepiston.

The width of the slot may advantageously be relatively wide since itneed only be separated by a small distance from the rack on one side andthe end of the piston on the other. Since leakage from the slot isprevented by the phenomena of high capillary attraction, extra spacedoes not have to be allowed to provide for a relatively wide highpressure oil "print" previously found necessary.

The fluid feed grooves along the longitudinal axis of the piston andspaced on opposite sides of the output rack operate substantially asdescribed and claimed in the Sheppard '063 patent but, in addition in mypiston, these grooves serve the important function of feeding theshallow capillary slots. Prints are formed along the grooves assistingin the balancing and cushioning action of the piston. The capillaryslots greatly improve reversibility or break loose capability of thepiston when they are filled with the high pressure oil. This isespecially effective in steering the vehicle under the most unfavorableconditions, such as with a fully loaded front axle in deep sand or mud.The maximum output torque needed under these conditions can be realizedsince (1) there is minimum resistance to movement of the piston due tofriction and slip stick effect, and (2) there is maximum high pressureoil to move the piston since any significant leakage is prevented. Thelength of the slots may vary and still be highly functional, but in thepreferred embodiment shown, the slots extend over substantially 120degrees of the circumference of the piston directly opposite the inletslot. However, the depth, width and length of the slots described hereinprovides the optimum balance to the piston within the cylinder andvirtually eliminates the slip stick effect along the rack side of thepiston, and thus is of substantial importance with respect to the morelimited aspects of my invention.

Complimenting the action of the capillary slots to improve reversibilityof the gear, and specifically to improve the reversibility of the valvewithin the piston, is my novel approach of providing the oil delivery ortransfer passage within the piston between the inlet slot and the valveby a bifurcated passage. The passage feeds the operational high pressureoil along the sides of the valve so that oil is quickly and efficientlyfed around the full periphery of the valve with approximately equaldynamic pressure. This arrangement provides a balance or hydraulicfloating of the valve within the valve bore just as the piston isbalanced with the cylinder bore of the gear housing.

The net effect of the improvements described is that the piston is ableto respond quicker and more efficiently when an input signal is receivedthrough the input shaft of the gear. Furthermore, the advantages havebeen attained while maintaining the non-productive leakage between thepiston and the cylinder and around the valve at an absolute minimum,thus making available the full volume of fluid for moving the piston tosteer the vehicle.

Both the balancing of the valve and the balancing of the piston are selfcorrecting. As the valve breaks loose or off center by action of theinput shaft, the pressure increase is the same in all lateraldirections, virtually suspending or floating the two moving parts, thatis, the valve and piston. On the piston, the combined force of theaction of the output pinion and the rack, the fluid pressure print alongthe fluid feed groove, and the high pressure oil traped in the capillaryslots offsets the force from the high pressure print around the inletslot. Simultaneously, delivery of the fluid to the sides of the valve,rather than at the top, balances the valve around its full periphery.

Still other objects and advantages of the present invention will bereadily apparent to those skilled in this art from the followingdetailed description, wherein I have shown and described only thepreferred embodiment of the invention, simply by way of illustration ofthe best mode contemplated by me of carrying out my invention. As willbe realized, the invention is capable of other and differentembodiments, and its several details are capable of modification invarious obvious respects, all without departing from the invention.Accordingly, the drawings and description are to be regarded asillustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional view taken along the center axis of aSheppard Power Steering Gear utilizing the new concepts of my invention;

FIG. 2 is a cutaway view along the side of the piston adjacent theoutput rack showing the capillary slots and the fluid feed grooves;

FIG. 3 is a plan view of the piston on the output rack side showing thefeed grooves and the connecting capillary slots;

FIG. 4 is a detailed cross sectional view taken along line 4--4 of FIG.3 at the end of the piston;

FIG. 5 is a cross sectional view taken along line 5--5 of FIG. 3 at thecenter of the piston showing the bifurcated oil delivery passage of thepresent invention; and

FIG. 6 is a cross sectional view taken along lines 6--6 of FIG. 3 andshowing the opposite end of the piston at a slightly different position.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A more complete description and, thus, a more complete understanding ofthe preferred embodiment of the invention can be realized by referenceto the several drawings described above. Beginning with FIG. 1, a powersteering gear 10 is shown in sufficient detail to provide the requisiteunderstanding of the invention. The basic parts in which changes havebeen made to realize my improvement can be seen. Thus, the basic partsof the steering gear include power piston 11, gear housing 12 formingcylinder 13 for the piston 11, bearing cap 14 at one end of the cylinderand cylinder head 15 at the opposite end, output pinion 16 on outputshaft 17 operated by output rack 18 on one side of the piston 11, andlow pressure exhaust chamber 19 surrounding the output pinion 16 forpassage of the low pressure fluid through output port 19a to the sump(not shown) and eventual return to the pump (also not shown).

A spool type distributing valve 20 is mounted within a central bore ofthe piston 11. The spool-type valve 20 within the piston 11 is fed withincoming high pressure oil through an inlet port 21 that cooperates withan inlet distributing slot 22. The valve 20 is threadedly engaged withthe input shaft 23 supported by a suitable bearing 24 by the bearing cap14. A high pressure seal 25 is provided on the shaft and an outer dirtseal 26 is mounted adjacent the splined end 27 that is designed toreceive the connection to the steering shaft of the vehicle. As is wellknown in the art, the input signal to the steering gear is in the formof rotation of the input shaft and through the threaded engagement withthe valve 20, the appropriate distribution of the oil moves the piston11 to effect steering through the rack 18, the output pinion 16 and theoutput shaft 17.

The main supply groove around the periphery of the bore within thepiston which houses the valve 20 is designated by the reference numeral30. This main distribution groove 30 is fed by a bifurcated deliverypassage 31, that opens into the slot 22. This passage is of particularimportance to my invention, and will be described in more detail later.

Annular grooves 35, 36 in the body of the valve 20 split the incomingflow into two equal portions when the gear is in the standby or balancedcondition. The flow continues through these peripheral grooves intoouter grooves 37, 38 formed in the piston bore. The outer grooves 37, 38communicate through respective exhaust passages 37a, 38a, to the exhaustchamber 19. Dual pressure feed passageways communicate with the landsbetween the annular grooves 30, 38 in the piston to feed the bearing capend of the piston with the high pressure fluid (see passageways 40, 40ain FIG. 4). Similarly, dual feed passageways going to the opposite endof the piston adjacent the cylinder head 50 provides pressurization tomove the piston in the opposite direction (note passages 41, 41a of FIG.6).

Reversing springs 42, 43 are mounted between the ends of the valve 20and the adjusting nuts 44, 45 threaded into the end of the valve bore,as best shown in FIG. 1. A complete operation of the valve and thedistribution of the oil and the operation of the valve reversing springs42, 43 can be more completely reviewed by reference to the previousSheppard '063 patent.

Still referring to FIG. 1, it will be realized that the high pressureoil coming into the inlet port 21 provides a lateral force against thepiston 11, which lateral force is concentrated along the slot 21 and theprint of the oil surrounding the slot as the oil trys to squeeze betweenthe mating cylindrical surfaces. When the piston moves, causing the rack18 to turn the output pinion 16, the meshing of the teeth tends to camthe piston 11 in the opposite direction. However, this opposite lateralforce is not equal to the hydraulic force pushing the piston 11 towardthe pinion 16. The fluid balancing and cushioning means of the presentinvention fills the need to offset the discrepancy.

In this respect, the objectives of the present invention are met byproviding a capillary slot 50 at the bearing cap end of the piston, anda similar capillary slot 51 at the cylinder head end of the piston.These slots are filled with high pressure oil through the longitudinallyextending feed grooves 52, 53 (see FIGS. 2 and 3).

The depth of these capillary slots at the ends of the piston iscritical. As previously indicated, I discovered that making the grooveslimited in depth to provide high capillary attraction between themolecules of the oil and the walls of the groove brings about thedesired high efficiency operation. Specifically, the slots 50, 51 arepreferably within the range of 0.001 inch to 0.0025 inch. At this depth,it has been found that a phenomena occurs that allows the molecules ofoil to be firmly retained or trapped within the slots 50, 51. When themolecules cannot escape, leakage of oil is minimized and the efficiencyis enhanced considerably.

As steering takes place, and higher and higher pressures occur in thegear, the pressure within the slots 50, 51 also increases so as tosubstantially offset the increasing pressure along the inlet slot 22.This is desirable since the lateral restoring force is thus selfregulating, and provides a piston that is suspended virtuallyfrictonless within the cylinder bore in a very advantageous manner.

Although the phenomena by which the oil is retained within the slots50,51 may not be fully comprehended at present, it is believed that theextreme shallowness of the slots allows the oil molecules to come intomore intimate contact with the bottom of the slots. Thus, the extent ofoil molecules stacked on top of the one or two thousandths inch layer orfilm of oil is minimal. Given the extremely close tolerances heldbetween the piston and cylinder itself, and the natural affinity of theoil molecules for the metal forming the slots 50,51 (capillaryattraction), the oil cannot escape. The bottom of the groove is machinedwithout polishing, which further improves the oil retainingcharacteristic of the slots. The balancing function is thus accomplished(1) with greater efficiency, and (2) withut significant leakage, whichis the realization of the best of both prior improvements in one powersteering gear structure.

Broadly then, one key to this aspect of my invention lies in making theslots 50,51 sufficiently shallow whereby the capillary action ormolecular attraction is enhanced to trap the oil to balance and cushionthe cylinder and at the same time prevent leakage.

The slots 50,51 are efficiently fed with high pressure oil through thegrooves 52,53 which taper toward the ends to the very shallow depthrequired in the slots 50,51. It has been found that the feed of oil tothe slots 50,51 is highly efficient and provides quick response so as toquickly increase the pressure and balancing force when an input signalis received and steering is necessary. Feed grooves 52,53 are providedwith pressurized oil continuously by being connected to the centralannular groove 30, as best shown in FIG. 4. The connection between thegroove 30 and the feed grooves 52,53 is made through transfer passages55,56 (see FIG. 5).

The slots 50,51 extend around approximately 120° of the periphery of thepiston 11. This further helps to maximize the balancing and cushioningeffect to the piston. The width of the slots for optimum performance isapproximately 3/16 inch.

A conventional blow down passageway 60 with spring controlled ballvalves at each end is provided and also connected to the centralpressure groove 30 (see FIG. 4). This passageway 60, thus providesdirect relief when the limit of travel of the piston 11 has beenreached.

Bifurcated delivery passage, generally designated by the referencenumeral 65, having individual flow passages 66,67 connects oppositesides of the central annular groove 30 (see FIG. 5) with the inlet slot22. Thus, the high pressure oil from the inlet groove 22 is immediatelypassed to the sides of the valve so as to quickly flow around theperiphery of valve 20. In this manner quicker access to the transferpassages 55,56 is obtained. By taking this more direct route to thepassageways 55,56 the oil flow to the slots 50,51 through the feedgrooves 52,53 is greatly improved, and starving, even under the mostunfavorable circumstances, is obviated.

The dynamic pressure of the oil around the valve 20 with this newdistribution concept is such that the valve 20 is virtually floated orbalanced in the bore of the piston, thus complementing the balancingaction of the piston. In prior designs, the high pressure oil cominginto the valve bore tends to push the valve toward the output pinionside of the gear. This tendency has been eliminated by my approach, thusgreatly improving reversibility of the valve 20.

There is efficiency gain of at least 5-6% in the power steering gear ofmy invention where the piston 11, as well as the valve 20, aredynamically balanced by the fluid pressure in the system, as describedabove. The molecules of oil within the very shallow slots 50,51 aretrapped by capillary attraction, and thus the tendency for leakage bysqueezing of the oil between the outer surface of the piston and theinner surface of the cylinder is obviated. Greater efficiency of thepower steering gear comes from thus being able to provide the maximumfluid flow and pressure for movement of the piston within the cylinder.

There are substantial benefits in terms of improving the reversibilityof the piston 11 within the cylinder 13 since the slip stick effect isprevented in the critical area at the ends of the piston 11 adjacent therack 18. Wear is reduced to an insignificant amount and the potentialfor binding of the parts is virtually eliminated.

In the disclosure there is shown and described only the preferredembodiment of the invention, but, as aforementioned, it is understoodthat the invention is capable of use in various other combinations andenvironment and is capable of changes or modifications within the scopeof the invention concept as expressed herein.

I claim:
 1. An improved piston and cylinder combination of a hydraulicpower steering gear comprising a housing defining a cylinder, a pistonslidably mounted within the cylinder; a slot formed in at least one ofsaid piston or cylinder at the sliding interface and open only at saidinterface, and means to feed pressurized hydraulic fluid adjacent theends of said cylinder to actuate said piston and to said slot to balanceand cushion the piston, said slot having a depth of approximatelybetween 0.001 and 0.0025 inch thereby being sufficiently shallow to setup high capillary attraction of the fluid molecules within the slot,whereby the fluid is trapped within the slot and leakage is prevented.2. The piston and cylinder combination of claim 1 wherein said slotextends at least partially around the piston adjacent the end.
 3. Thepiston and cylinder combination of claim 2 wherein is provided anadditional slot extending at least partially around the other end. 4.The piston and cylinder combination of claim 3 wherein said feed meansincludes grooves extending longitudinally along said piston andconnecting with said slots.
 5. The piston and cylinder combination ofclaim 1 wherein said slot is formed on said piston and extends aroundapproximately 120° thereof.
 6. The piston and cylinder combination ofclaim 1 wherein said feed means includes a valve within a bore in thepiston, an inlet to supply pressurized fluid to said valve, transferpassage means on the side of said valve opposite to said inlet, and abifurcated delivery passage communicating with said inlet to direct saidfluid around said valve substantially directly to said outlet passage,said fluid from said bifurcated delivery passage flowing around saidvalve so as to substantially balance the valve within said bore.
 7. Anintegral hydraulic power steering gear having a reciprocating piston,means to feed pressurized hydraulic fluid to said gear to actuate saidpiston, a valve to selectively distribute pressurized fluid to the endsof the cylinder for steering action, means to provide an outside lateralforce on said piston, and at least one shallow slot formed in saidpiston on the side opposite said force and said slot open only towardthe cylinder wall of said cylinder, and means to feed pressurizedhydraulic fluid to said slot to balance and cushion said piston, saidslot having a depth of approximately between 0.001 and 0.0025 inchthereby being sufficiently shallow to set up sufficient capillaryattraction of the fluid molecules within the slot so that the fluid istrapped within the slot and leakage is prevented.
 8. The integral powersteering gear of claim 7 wherein said slot extends substantially 120°around said piston.
 9. The integral power steering gear of claim 8wherein said slot is formed at one end of the piston and an additionalslot is provided at the opposite end of the piston.
 10. The integralpower steering gear of claim 8 wherein the slot is machined in thesurface of said piston, the bottom of said slot being unfinished toincrease the attraction of the oil.